Online gadget resellers are currently experiencing a huge smartphone trade-in surge due to tomorrow’s unveiling of the next-generation iPhone, but they also warn that today’s attractive prices will not last forever (or for another 24 hours, for that matter).

Gazelle, which is one such website that noticed the ballooning amount of trade-ins, said it witnessed a 600 percent increase over the last 30 days, when compared to the 30 days prior to the 4S launch, but the company’s chief gadget officer, Anthony Scarsella, said the business growth shows “just how excited consumers are about the iPhone 5.”

Gazelle offers multiple ways to earn moolah for iPhone trade-ins, such as a check, PayPal or Amazon gift card, but selecting the Amazon option will grant 5 percent more toward the quoted value. The service currently offers a “good” iPhone 4 8GB (Black) – Verizon roughly $140, while an iPhone 4S 64GB (White) – Verizon in the same condition goes for $350.

Values for the iPhone 4 and iPhone 4S have slumped nearly $60 since last month, so Gazelle’s CEO recommends acting now before the next-generation iPhone makes it debut. Scarsela elaborated:

After the Apple announcement, we’re expecting to see prices start to drop dramatically. In the past, we’ve seen prices begin to drop right after the announcement and continue to decline for about a month before they level off. We most definitely expect to see that happen with this announcement.

Glyde’s chief executive officer, Drew Lieberman, contended that his peer-to-peer marketplace always reflects any market changes, but he is confident a 16GB iPhone 4s will “still net at least $200″ after tomorrow’s Apple event. While Glyde does not have a typical trade-in program, it does make it easier for consumers to buy and sell used electronics.